Real-time-sensitive data transmission can be substantially described by two parameters. For one, the data must have been received completely by the recipient, and for another the data must have also been received within a predetermined time.
A distinction can be made here between so-called soft real-time communication and hard real-time communication.
Video data transmission and/or audio data transmission is understood, for example, as being soft real-time communication.
Unlike soft real-time communication, wired systems are generally relied upon for hard real-time communication, particularly if the demands for the probability of loss for data to be transmitted in a given time span are high, which is to say that the probability of loss is very low.
For instance, in the field of automobile automation, the requirements for the probability of loss for control data lie in the range of 10−6 to 10−10. Furthermore, transmission within a few milliseconds down to 100 microseconds is required in this range. It should be noted here that the actual payload size of the data to be sent tends to be rather small and is often only 100 bytes or fewer.
In order to meet these demands, communications systems that are based on optical data transmission have been used in order to fulfill the particularly high demands placed on the probability of loss in relation to the available time span.
This approach is disadvantageous, however, since wired transmission is inflexible and generally associated with high costs for reinstallation upon modification of system parts. Furthermore, it's been shown over and over that wired systems are damaged again and again during construction projects, resulting in downtime of the system.
Although the use of wireless systems has been proposed in isolated cases, no solution has yet been presented that resolves the most important problem of wireless systems, including with respect to the demands for real-time-sensitive data.
The most important problem here is that the wireless communication channel must be designed such that the stochastic characteristics of the wireless communication channel remain inconsequential.
In doing so, it must be considered that the quality of a wireless communication signal can vary greatly within a short period of time. This can for example be “promoted” by noise sources, but equally cancellation can be promoted by moving metal parts that cause reflections.
Particularly in the field of industrial automation, there are such sources of interference known that can contribute to massive fading of the signal.
In other fields as well, such as in the automobile industry, increasingly attempts are made to introduce wireless transmission in order to minimize the effort and expense of constructing wire harnesses and simultaneously save weight.
Here, too, essentially the same statements can be made as before for industrial automation.
Although first steps are being taken to make wireless communication useful in this field as well, the previous approaches have not been suited to the demands of real-time communication in terms of hard real-time communication.
This is based on the fact that, for one, the general ability to process fluctuations of the wireless channel does not necessarily mean that the previously used measures are also suitable for effectively counteracting fluctuations in the scope of (hard) real-time communication.
Highly developed error correction codes, for instance, cannot be used due to the highly limited transmission time, since these are very time-intensive both during encoding and decoding. Only with special, costly hardware would such an application be conceivable.
Moreover, transmission techniques that are based on instantaneous channel status information cannot be used, since no time is generally available for channel acquisition. There is generally no time for retransmission, either, since this retransmission requires active feedback on the part of the receiver via an incorrectly received message, and the retransmission required as a result must also occur within the given time.